At least 10,000 people been killed since fierce fighting erupted in the capital Juba in December

By Aaron Maasho

ADDIS ABABA, Aug 4 (Reuters) - South Sudan's warring sides opened the latest round of talks on Monday as regional mediators warned time is running out for a country which aid agencies say is on the brink of a famine.

At least 10,000 people been killed since fierce fighting erupted in the capital Juba in December pitting government forces of President Salva Kiir against supporters of Riek Machar, his former deputy and long-time political rival.

The conflict has reopened deep ethnic tensions between Kiir's Dinka and Machar's Nuer, in a country which only won independence from Sudan in 2011.

Negotiations have rumbled on for months with scant progress in the Ethiopian capital. The United States and the European Union have slapped sanctions on military leaders from both sides after the first ceasefire in January swiftly crumbled.

Kiir and Machar signed a second ceasefire deal on May 9, and agreed to a 60-day timeline in which they were supposed to thrash out details for the formation of a transitional government. Further progress, however, has been marred by disagreements on who can participate at the talks.

Each side has accused the other of violating the May ceasefire but large-scale fighting has been rare, partly due to the onset of the rainy season which restricts movement in the vast African country that boasts only a handful of paved roads.

Western diplomats say they are losing patience with both sides and warn a "man made" famine is looming.

The Inter-Governmental Authority on Development (IGAD) - a regional grouping brokering the negotiations - opened the latest round of negotiations by reminding both sides that it was ready to take measures against them if no progress is made.

"Promises are not enough. We must truly deliver," Seyoum Mesfin, IGAD's chief mediator of the talks, said in a speech. "The region will not tolerate any delay."

Ethiopian Prime Minister Hailemariam Desalegn, whose country is current chair of IGAD, said in July the bloc would not rush to impose sanctions, but that he believed the latest round of talks would be a "last attempt" before measures are taken.

One IGAD official told Reuters the warring sides will discuss how to implement a ceasefire deal which would also detail rebel and government positions in the battlefield. An agreement on the issue would then pave the way for withdrawal of Ugandan troops who have propped up Kiir's government.

Aid agencies say South Sudan could be headed for the worst famine since the mid-1980s, when malnutrition swept through East Africa and killed over a million people.

Four million people - or more than a third of the country's population - are now believed to be facing emergency levels of food security. (Reporting by Aaron Maasho; editing by Drazen Jorgic)